This invention relates to a method for obtaining the desired properties such as mechanical or physical properties incident to curing thermosetting polymeric or elastomeric material. The invention is particularly useful for establishing and obtaining precisely desired properties of such materials.
Conventionally, in the past, it was the usual practice to use empirical formulae and rely on expertise developed from past experiences for cure control when molding synthetic or natural thermosetting polymers or elastomers, hereinafter referred to for brevity, as rubber material. A complete cure curve for obtaining properties of rubber material during curing could not be predicted. For example, it is a widely-accepted general rule that the rate of cure must be doubled for each 10.degree. C. increase in cure temperature. It is also assumed that the cure time must be increased by 5 minutes for every 1/4 inch thickness of rubber in the mold. As disclosed, for example, in U.S. Pat. No. 3,819,915 and British Pat. No. 1,293,941, an Arrhenius equation is set forth for calculating the cure time but, unfortunately, the equation cannot be utilized to predict or establish properties from a complete cure curve for the rubber during curing.
Understanding and predicting cure kinetics of elastomeric or thermosetting materials are useful both in the manufacturing process and performance and reliability of the manufactured rubber product. In most polymeric systems, however, the cure reactions are quite complex such that the general practice discussed previously cannot be utilized to explain and predict changes of the physical and mechanical properties of the rubber during the cure reaction. In the field of this invention, cure kinetics involved synthetic or natural rubber which may be compounded or have blended with it in the process of manufacture, various materials, such as pigments, fillers, waxes, oil, fibers, to obtain a wide range of properties, and may be combined with various structural materials, e.g., with textile fibers. Studies of cure kinetics are extensive but limited to the method of calorimetry such as differential scanning calorimetry or differential thermal analysis. The state of cure is not directly related to the physical, mechanical or rheological properties of the rubber. Kinetic models developed from the calorimetric method cannot be utilized to predict properties such as the viscosity and dynamic modulus. These properties are useful for determining the manufacturing operation and the end product performance of the rubber. Most polymeric systems of rubber do not behave according to the generally-accepted rules because the phenomenon of heat transfer and cure is oversimplified. The cure rate of the polymeric system depends on the basic polymers, curative, cure temperature and filler loading. Cure kinetics in the present invention are based on a non-equilibrium thermodynamic fluctuation theory of chemical relaxation. Known methods for predicting the viscosity and modulus during the cure of rubber can only be utilized to predict a narrow range of data during cure and polymer-filler interactions are excluded.